It is customary to perforate a completed oil well to obtain production from the adjacent formations. The ideal perforation is relatively large in diameter and penetrates as deep as possible. A large and deep hole best provides a flow path from the producing formation. This flow path must typically extend from the formation through the cement around the casing and also through the casing. A relatively large and deep perforation is therefore desirable to reduce pressure drop along the flow path to assure proper production from the formation into the borehole for collection in the cased well. There are however, physical constraints on obtaining such large and deep perforations.
Ordinarily, a tubing string is placed in the well within the casing. The size of the tubing string limits the size of the through-tubing perforating gun assembly. A popular tubing string has a nominal size of 23/8" which provides an ID of 1.995". When this tubing string is installed along with the necessary completion components (e.g., nipples, sliding sleeves, safety valves, etc.), the minimum ID clearance is normally decreased from the nominal measure just stated. It is not uncommon to have an ID of 1.875" or even as low as 1.781". Given these dimensions, the diameter of the perforating gun assembly is thus limited. Common through-tubing perforating gun assemblies are typically 1 9/16" OD with a hollow carrier, or 1 11/16" OD if using an expendable carrier. Many circumstances prevent the use of an expendable carrier, including operation in a well that is unduly hot, one with hostile downhole fluids, or where the casing is weak or unsupported. In the event that a 1 9/16" OD hollow carrier gun is required, it typically is installed with a 3.0 gram explosive charge. Such an explosive charge will provide (in accordance with the API RP-42 concrete test) an entrance hole of 0.24" diameter and penetration of 6.17". Regrettably, firing of the perforating gun provides swelling to about 1.68" OD. While this swelling can be acceptable in some circumstances, a larger and deeper perforation obtainable by using larger charges is much more desirable.
Consider the possibility of using shaped charges of the size normally conducted in a 2" OD gun. These shaped charges will carry much more powder, typically 7.5 grams. Utilizing the same test standard, they will provide an entrance hole diameter of 0.29" and about 10.30" penetration. The charges (normally run in 2" guns) can be conveyed in a thin-walled carrier that will pass through 1.781" restrictions. A typical carrier OD might be 1.750". While such a large perforating gun assembly can be conducted down the tubing string, it is difficult to retrieve because of swelling, and possibly even splitting, the thin-walled gun. The size (after swelling) is so large as to prevent retrieval through a typical 23/8" completion string with the necessary installed apparatus.
It is desirable to drop the perforating gun assembly after operation so that retrieval of the support equipment can be accomplished without sticking the portion of the equipment which swells after operation.
It has been proposed to use frangible screws or other fasteners which hold the perforating gun assembly together. Ideally, one can size a frangible screw or pin which would timely shear. This requires balancing in that it must be sufficiently weak to shear in a wide range of circumstances and yet it must be strong enough to avoid shearing when the tool is run down the tubing string to the desired depth in the well. An example of such a structure is shown in U.S. Pat. No. 4,496,009 which utilizes an unfocused charge to assure that the shear screws or pins are broken. This has the drawback of requiring the use of a second kind of shaped charge in addition to those used for perforating. Furthermore, the unfocused shaped charges must be located as far as possible remote from the casing to prevent unintended casing damage on detonation. Such clearance is typically not available in limited clearance situations. It is therefore better to avoid frangible pins or screws and thereby avoid use of the unfocused charges.
The present apparatus enables disconnection of the perforating gun assembly to drop a portion thereof to enable retrieval of the remaining portion of the equipment. This apparatus utilizes a plug positioned selectively in a set of locked collet fingers. The plug operates as a piston. When the perforating guns are fired, the piston is then driven upwardly, the shift thereof releasing the collet fingers so that the entire assembly breaks into two parts. The device is further constructed so that outgassing of the shaped charges does not pose a problem during running into the well even in the presence of elevated temperatures.
This apparatus is summarized as a perforating gun assembly adapted to be run in a small ID, the assembly including a firing head at the upper end supporting a detent housing. A pressure responsive piston is deployed therein and has an upstanding stem, the stem collaborating with a set of spring operated plungers which lock the piston in an elevated position after firing. When the assembly is run into the hole, the piston is moved to the down position where it locks a set of collet fingers. The collet fingers join with a collet body which in turn supports the remainder of the perforating gun assembly therebelow. It includes a carrier with one or more shaped charges therein. Upon detonation, the piston is forced upwardly and locked in the elevated position after movement. When it moves, it releases a set of collet fingers and thereby enables the collet body to pull free. It drops away by gun blast and gravity and carries the carrier with it. It should be noted at this stage that the carrier is expanded or possibly split as a result of swelling at the locations where the shaped charges explode in the carrier.